To Lecture Notes

IT 231 -- 7/30/12

Review Questions

1. Which of these are legal Ruby variable names?.
   

   abc    b768    4slots   First Name 
   
   _n_    hook&ladder    numberOfCustomers 
   
   number-of-customers    PreferredCustomer 
   
   an_extremely_long_variable_name_if_you_ask_me 
   
   Answers: 
   
   abc (legal)   
   
   b768 (legal)  
   
   4slots (illegal, starts with a digit)   
   
   First Name (illegal, can't have embedded space)   
   
   _n_ (legal)  
   
   hook&ladder (illegal, can't use &)  
   
   numberOfCustomers (legal, but not recommended Ruby syntax)
   
   number-of-customers (illegal, can't use dash in variable name)
   
   PreferredCustomer (legal as a class name, which uses upper camel casing)  
   
   an_extremely_long_variable_name_if_you_ask_me (legal, but should be shorter
   to be practical)
   

2. List some Ruby operators. Ans:
   

   Arithmetic operators: +  -  *  / 
   
   Comparison operators: <  >  <=  >=  == !=
   
   Assignment operator:  =
   

3. How can you convert an integer into a string?
   Ans:   n.to_s
    
4. How can you convert a string into an integer?
   Ans:   s.to_i
    
5. What is the difference between print and puts in Ruby?
   Ans: print allows you to print multiple items at a time like this:
   

   print s, "  ", t, "  ", 23, "\n"
   

   puts prints a single string like this:
   

   puts s + "  " +  t.to_s
   

   puts automatically adds "\n" at the end of the string, so it's not necessary to explicitly include it.
    
6. Explain the difference between Ruby classes and objects.
   Ans: A Ruby class forms the blueprint or design for an object. If A is the class name, then x = A.new creates a new object. You can create as many objects as you wish from one class.
    
7. Explain what the Ruby dot operator does.
   Ans: It calls a method. Methods can either be class methods (called from a class like t = Time.new or object methods (called from an object like t.hour).
    
8. What will this Ruby line print?
   

   print Time.now.month
   

Ans: It will print 8 if August is the current month.
 

9. Try out these Ruby statements in IRB. What do you think is happening?
   

   a = "Jeremy"
   b = 342
   print "My friend #{a} owes me $#{b}.\n"
   

Ans: This is an example of variable interpolation. #{a} means insert the value of the variable a at this location in the string.

10. Change the double quotes in Question 10 to single quotes. Explain.
    Ans: Variable interpolation and escape characters only work with double quotes, not single quotes.
     
11. Write a Ruby line that prints a random number from 0 to 10 with equal probability.
    

    print rand 11
    

    
     
12. Write a Ruby line that assigns to employee_id, a random number 1001 to 1999 with equal probability.
    

    employee_id = 1001 + rand 999
    

    
     
13. What is the value of a after these Ruby statements?
    

    x = 5
    if x + 2 > 9
        a = "yes"
    else
        a = "no"
    end
    

Control Structures

• Some Ruby control structures are if..else statements and while loops.
   
• A Ruby/JavaScript Control Structures Dictionary
   
• Look at Examples 17 through 21.
   
• The mod operator % returns the remainder from integer division.
   
• For example, consider the integer division problem 17 / 3.
   
• 3 goes into 17 5 times, with a remainder of 2. Therefore the Ruby script
  

  r = 17 % 3
  print r, "\n"
  

  outputs 2.
   
• The mod operator is used in Example 20.
   

Modifying Ruby Code to Accept User Input

• To modify a Ruby command prompt script to accept user input, use STDIN.gets to read keyboard input and the String chomp method to remove the trailing "\n" from the input string.
   
• For example, change
  

  c = 20
  f = 9 * c / 5 + 32
  print "The Fahrenheit temperature is #{f}\n"
  

  to

  print "Enter a Celsius temperature: "
  input = STDIN.gets
  c = input.chomp.to_i
  f = 9 * c / 5 + 32
  print "The Fahrenheit temperature is #{f}\n"
  

• Modify Examples 13, 17, and 20 to accept user input from the keyboard.
   

Modifying Images with Image Editing Software

• Resizing an image with MS Paint
   
  
  Right Click on image name in MS Explorer >> Select open with Paint. MS-Paint will open. Select Resize Image >> Set the horizontal and vertical boxes to a percentage less than 100 to shrink the image. Save the image.
  
  
• Cropping an image with MS Paint  
   
  
  Right Click on image name in MS Explorer >> Select open with >> paint. MS-Paint will open. Click the Select Button >> Use the mouse to select the rectangular area of the image to keep >> Click the crop button. Save the image.
  
  
• Lightening an image with Adobe Photoshop  
   
  
  Invoke Adobe Photoshop. Select File >> Open >> Browse to desired imaage. Select Image >> Adjustments >> Hue-saturation. Move the Lightness slider to the right. Click OK. Save the image.
  
  

Project 4

• Look at the Flintstones Example (Example 22).
   
• Look at Project 4.

The Array Class

• An array is a Ruby object that contains other ruby objects.
   
• Look at the Array Class UML Diagram.
   
• Three ways to create and populate an Array object:
  

  # Use the new method to create an empty array.
  # Use the << method to append items to the array.
  pets = Array.new
  pets << 'dog'
  pets << 'cat'
  pets << 'parrot'
  pets << 'goldfish'
  
  # Use the shortcut method:
  pets = ['dog', 'cat', 'parrot', 'goldfish']
  
  # Use the new method with a multiplication factor:
  a = Array.new(3, 'dog')
  

• Question: what is the value of pets[2]? Hint: an array starts counting its objects at the left with zero.
   
  Ans: It is 'parrot'. The indices always start counting with 0 from the left: a[0] is 'dog', a[1] is 'cat', a[2] is 'parrot', and a[3] is 'goldfish'.
   
• Write Ruby statements to test some of the methods of the Array class.
   
• An array is used in a Rails website to hold resources that are retrieved from or inserted into a database table.

Ruby each Statement

• Use each method to repeat a code block for each item in a range or array:
  

  # Example 1
  a = [3, 4, 5, 6, 7]
  a.each do |i|
     puts i + 1
  end
  
  Output:
  4
  5
  6
  7
  8
  
  # Example 2
  sum = 0
  r = [3, 4, 5, 6, 7]
  r.each do |i|
     sum = sum + i
  end
  puts sum
  
  Output:
  25    (The sum of the numbers in the array.)
  
  # Example 3
  pets = ['fido', 'felix', 'jerry']
  pets.each do |p|
      puts "I love my pet " + p + "."
  end
  
  Output:
  I love my pet fido.
  I love my pet felix.
  I love my pet jerry.
  

Practice Problems

1. Create a Rails model GradeRecord with fields stud_id: integer, last_name:string, first_name:string, course_number:string, course_name:string, prof_name:string, credit_hours:integer, grade:string. Append this Ruby seed code to the file db/seeds.db. You can then seed the database with startup values by typing
   

   rake db:seed
   

   Now start the Rails console and load all of the records from the database into an array with

   a = Person.all
   

   Use for..each statements or each methods to print the first and last names for all the grade records:
   

   a.each do |g|
     print "#{g.first} {g.last}\n"
   end
   

2. Create a Rails model that contains Student objects with fields name (string), year (integer), gpa (float), fulltime (boolean). Then use the Rails console to:
    
1. add three new Person objects to the database.
    
2. shut down the server and start the Rails console.
    
3. load all of the records from the database into an array with

   a = Person.all
   

4. Use for..each statements or each methods to print the names of all the students to the console.
 
3. Write a Ruby script that inputs the course score and outputs the course grade.
   A == 90..100, B == 80..89, C == 70..79, D == 60..69, F == 0..59. Use an if..elsif..end statement.

Review for Midterm

• Look at these items on the Exam Info Page:
   
  
  Midterm Review Guide
   
  Midterm Review Questions 1
   
  Midterm Review Questions 2
   
  Code with Errors to Correct.